This invention relates to the internal coating of a heated tube and in particular to a coating process wherein fusible particles are transported through the tube in a flow of air.
It is frequently advantageous to coat the inside of steel tubing with nonmetallic linings. Properly selected linings can protect the tube against erosion or corrosion. A coating surface which is smoother than that of the tubing itself can result in a decreased pressure drop of fluid flowing through the tube. For such a purpose materials have been used such as polypropylene, chlorinated polyether, various polyester, certain epoxy and vinyl polymers, polyethylene, polytetrafluoroethylene, and nylon. These materials may be either thermoplastic of thermosetting, and are normally applied to the tube after it has been heated.
It is desirable that the internal coating be free from voids or pin holes. It also should have a uniform thickness and a smooth surface. The method used to coat the tube should be both reproducible and simple in operation.
It is known to heat and rotate the tube to be coated so that fusible particles conveyed in an air stream and injected into the tube will stick to the surface and form a coating thereover. There have been many methods, along with apparatus therefor, suggested involving this general concept of coating a tube.
Fluidized beds have been suggested as a supply source for the fusible particles from which location they are carried through the tube in a stream of air. This is generally a continous flowing stream with excess particles collected at the other end. In some cases the apparatus provides a means for reversing flow through the tube from a second fluidized bed to promote uniformity of coating of the tubing.
It has been suggested to use a timed pulse of flow carrying the fluidized particles followed with a flow of particle-free air scavenging the tube.
These various methods in general require a significant excess of particle flow and/or timing of the cycle. Effectiveness of this operation requires that the timer be consistent and readily adjustable and that there be no leaks in the system whereby the timing would not represent an appropriate particle flow. A uniform, preferably thin and smooth coating, remains difficult to obtain in a consistent and reproducible manner.